FISH AGE, LENGTH, TROPHIC FEEDING AND LAKE SURFACE AREA AS PREDICTORS OF MERCURY CONCENTRATIONS IN PREDATORY FISH IN THE MACKENZIE RIVER BASIN

Recent studies assessing fish stocks in the Mackenzie River Basin, Northwest Territories, have discovered predatory fish in many lakes have elevated mercury concentrations. Specifically, mercury concentrations in lake trout, pike, and walleye exceed the 0.5 ppm guideline for the commercial sale of fish and the 0.2 ppm guideline for frequent consumers of fish. Consumption advisories have been issued. We have been investigating the factors contributing to these elevated mercury concentrations. High mercury concentrations are associated with predaceous feeding and with larger fish. Moreover, in general, fish inhabiting lakes in the NWT are relatively and slow growing. Thus, mercury levels tend to be higher than in the same-size species in northern Alberta and Saskatchewan. Lake surface area is important for pelagic fish such as lake trout and whitefish with higher mercury concentrations associated with fish living in smaller lakes. A weaker relationship is observed for pike, a littoral zone species, and for walleye, which inhabits both environments. Lake surface area may be important because of the increasing importance of the littoral zone in mercury methylation rates as lake area diminishes.